1. Field of the Invention
The present invention is related to an illuminating optical system and a projection display apparatus. More specifically, the present invention is related to an illuminating optical system that passes light output from light sources through a rod integrator to uniformize the distribution of light intensity and outputs the uniformized light, and a projection display apparatus that employs the illuminating optical system.
2. Description of the Related Art
Conventionally, there are known projection display apparatuses that modulate light beams output by light sources with light modulating elements such as DMD's (Digital Micromirror Devices) such that the light beams represent image information, and project the modulated light beams onto screens to display the image information.
In such projection display apparatuses, density fluctuations and color fluctuations are generated in image information displayed on screens if there are fluctuations in the distributions of light intensity of illuminating light beams that illuminate the light modulating elements. Therefore, there is demand to uniformize the light intensity distributions of the illuminating light beams.
Rod integrators are known as optical elements which can be employed to uniformize the distributions of light intensity of illuminating light beams. Rod integrators uniformize light intensity distributions by passing light beams that enter thereinto along a unidirectionally extending optical path while causing the light beams to reflect therein. Rod integrators are capable of forming the light beams input thereto into the cross sectional shape of the rod integrators and outputting light beams having uniformized light intensity distributions.
In rod integrators that output illuminating light beams (illuminating light) of which the light intensity distributions have been uniformized, the uniformity of the light intensity distributions of the illuminating light beams can be improved as the lengths of the rod integrators becomes greater in relation which the diameters thereof. This is because the number of times that light reflects within the rod integrators as they propagate therethrough increases as the lengths of the rod integrators become greater.
The light emitting surfaces of rod integrators and light modulating elements are placed in an optically coupled relationship, and the light modulating elements are illuminated by the illuminating light beams of which the light intensity distributions have been uniformized. Thereby, the illuminating light beams are modulated by the light modulating elements, become light beams that represent image information, are projected onto a screen, and display the image information on the screen without density fluctuations or color fluctuations.
Solid rod integrators formed by transparent members and hollow cylindrical rod integrators having mirrors as interior surfaces are known as rod integrators that uniformize the light intensity distributions of illuminating light beams.
Further, an illuminating optical system that combines a plurality of light beams output from a plurality of light sources using a cross prism, then passes the combined light beams through a rod integrator to form illuminating light beams is known (refer to Japanese Unexamined Patent Publication No. 2002-189263). In addition, an illuminating optical system that passes a plurality of light beams output from a plurality of light sources through rod integrators individually, then combines the light beams which are output from the rod integrators to form illuminating light beams is known (refer to U.S. Patent Application Publication No. 20060170884).